x-ray spectroscopy of diffuse galactic interstellar matter with chandra: the si k edge structure in...

X-Ray Spectroscopy of diffuse Galactic Interstellar Matter with Chandra:

The Si K Edge Structure in Galactic Bulge LMXBs

Norbert S. Schulz

Massachusetts Institute of Technology Kavli Institute for Astrophysics and Space Science

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Chandra HETG Legacy Projects: X-Ray K-edge Fine Structures

Boston, Chandra 15 yr, November 21st 2014

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Evolution & Recycling of Matter

Chemical Abundance in ISMs:

Elements in the X-Ray band (1.5 – 45 A): C, O, Ne, Mg, Si, S, Ar, Ca, Fe, Ni

Relevant Molecules (or not): CO2, CO, O2, H2O, CsH6O, CH2 O2

SiO2 , SiO44- deriv. (silicates)

Si3N4 deriv. (nitrides) Dust: Structure & Depletion

Ionization Fractions:Ionized Abundances

Star Formation Rates

Missing matter

Dynamics

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X-Ray Surveys: overview

Principle: Backlighting with bright X-ray continuaAdvantage: Long-range view through Galaxy Cover all ISM phases: C , O , Fe L, Ne Cold/Warm phase: Mg, Si, S, Ar, Ca, Fe, Ni Galactic Halo absorption ISM Studies in other Galaxies

Requirements: Spectral resolution > 1000, bandpass 0.1 - 8 keV

Sources: CVs, LMXBs, BH Binaries, AGN < 100 available for Chandra/XMM exposures > 100 available for Astro-H exposures > 1000 available for Athena and future Explorer

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X-Ray Surveys: previous/on-going X-ray studies

Low Z Edges: C K , O K, Fe L, Ne K

Carbon K edge: ACIS Contaminant (H. Marshall)

Nitrogen K Edge: Sant’Anna et al. 2011

Oxygen K Edge: X 0614+091 (Paerels et al. 2001) LMXB Survey (Juett et al. 2004) Cyg X-2 (Takei et al. 2002) Cyg X-1 (Schulz et al. 2002) XTE J1817-330 (Gatuzz et al. 2013) Neon K Edge: LMXB Survey (Juett et al. 2006) X 0614+091 (Paerels et al. 2001 Schulz et al. 2010) XTE J1817-330 (Gatuzz et al. 2013)

Ne/O resonance absorption: LMXB survey (Yao&Wang 2005, 2006) LMC X-3 (Wang et al. 2005) Cyg X-2 (Yao et al. 2009)

TBNEW in XSPEC: Wilms, Schulz, Nowak et al. 2013

High Z Edges: Mg K, Si K, S K, Ar K. Ca K, Fe K

Magnesium K Edge: GX 13+1, GX 5-1, GX340+0 Ueda et al. 2005

Silicon K Edge: GRS 1915+115 (Lee et al. 2002) GX 13+1…. (Ueda et al. 2005)

Sulfur K Edge: GX 13+1…. (Ueda et al. 2005)

Argon K Edge:Calcium K Edge:

Iron L & K Edges: Cyg X-1 (Schulz et al. 2002) X 0614+091 (Paerels et al. 2001) MCG 6-30-15 (Lee et al. 2001) GRS 1915+115 (Lee et al. 2002) Cir X-1 (Schulz & Brandt 2002) GX 301-2 (Watanabe et al 2003) Dust (Lee & Ravel 2005) ISM Dust (Lee et al. 2009)Nickel K Edge:

TBDUST in XSPEC Wilms, Lee et al. 2013

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X-Ray Surveys: Distribution of Chandra Sources

O, Fe (L), Ne Cyg X-1, Cyg X-2, GX 339-4,GX 9+9, 4U1820-30, 4U1636-534U1735-44, XTEJ1817-33, Ser X-14U 0614+091

Mg , Si , S , Ar , Ca , Fe (K)GX 5-1, GX 13+1, GX 9+1, GX 349+2GX 340+0, GX 17+2, 4U 1705-44,4U 1728-34, 4U 1624-49

Galactic Disk Galactic Bulge

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X-Ray Surveys: Previous Results on Oxygen – Dust vs. Ionization

Pinto et al. 2013

Gatuzz et al. 2013

Highlights:

A Chandra survey (Juett et al. 2004) of K edges measured the O K edge at 22.89+/-0.02 A and resolved resonance absorption lines for O I – IV with ionization fractions of 5 – 10 %.

Yao&Wang (2006) detect strong absorption from O VII and O VIII from the hot ISM phase in 4U 1820-30

Kaastra et al. (2009) measure abundance and absorption from O, Fe, and Ne towards the Crab and suggest dust contributions

De Vries&Costantini (2009) in an XMM-Newton RGS study in Sco X-1 detect EXAFS at the O K edge

Gatuzz et al. (2013) presented the most detailed modeling of the O K edge region in XTE K 1817-330 with neutral and ionized oxygen without dust contribution

An XMM-RGS survey by Pinto et al. (2013) of bright LMXBs provided dust contributions in the O K edge region up to 20%In GS 1826-238 Pinto et al. (2010) deduce dust contributionsbetween 20-40%.

Garcia et al. 2014 (submitted) re-analysed and remodeled all bright LMXBs in the Chandra archive and find absorption to be fully con-sistent with neutral and ionized oxygen and no dust.

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X-Ray Surveys: Previous Results on Neon Highlights:

A Chandra survey (Juett et al. 2006) of K edges mea-sured the Ne K edge at 14.31+/-0.02 A and resolved resonance absorption lines for Ne II, Ne III with ion-ization fractions of 5 – 10 % . Miller et al. (2004) hadpreviously identified these in GX 334-4.

Yao&Wang (2005) detect strong absorption from Ne IXof the hot phase of the ISM and deduce larger columnsat higher Galactic disk scale heights

Yao et al. (2006) constrain O VII and Ne IX columnsin the Galactic halo to < 5 x 1015 cm-2, less than 3% of the Galactic disk contribution.

Pinto et al. (2010) detect possible Ne VIII absorption at 13.7 A

Kaastra et al. (2009) in the Crab pulsar and Pinto et al. (2010)In GS 1826-239 find overabundance of Ne by a factor of 1.7 to solar

Juett et al. 2006

Yao & Wang 2006

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X-Ray Surveys: Previous Results on Iron L

Highlights:

Paerels et al. (2001) in X0614+091 detect Fe L2 and L3 edges with theChandra LETG, Schulz et al. 2002 in Cyg X-1 and Lee et al. in MGC6-15-30 model the Fe L edges using laboratory measurementsfrom Kortright&Kim (2000).

A Chandra survey (Juett et al. 2006) of K/L edges used this edge and various other models determine the Fe L2 and L3 edges at 17.26+/-0.02 A and 17.51+/-0.02 A, respectively, and conclude that that edges are a result of various dust contributions Yao et al. (2006) detect Fe VII absorption in 4U 1820-30 and find that the Ne/Fe abundance ratio is solar and all of the dust grains in the interstellar medium have been destroyed in the hot phase of the ISM.

Lee et al. (2009) showed that differences in L edge shape can be directly used to distinguish iron dust compounds assuming that all iron is bound in dust grains. Synchrotron measurement produced cross sections from a variety of Fe dust compound that match the Chandra measurements.

The fact that all or most Fe is bound in dust compounds has been shown by various studies (Draine 2003, Jenkins 2009) and emphasized in most high-resolution X-ray studies to date (Juett et al. 2006, Costantini et al. 2012, Pinto et al. 2013)

Juett et al. 2006

Lee et al. 2009

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X-Ray Surveys: Previous Results on Silicon Highlights:

In an Chandra HETG observation of GRS 1915+105 Lee et al. 2002 found traces of near and far edges structures in the Si K edge

Ueda et al.(2005) measured XAFS in the Mg K, Si K and Si K edges of three Galactic X-ray binaries and concluded that in the case of silicon most materials exists in silicates.

Through special data selection all of the observed edge structure was modeled as dust

The ISM edge was observed between 1.84 and 1.85 keV

The near edge absorption peak was identified as from SiO2

Si was determined to be overabundant in all thee sources and columns were measured to be well in excess of 1023 cm-2

GX 13+1 GX 5-1

GX 340+0

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Laboratory Measurements: Si K edge Li et al., Phys. Chem. Minerals, 1995: Silicon K-edge XANES Spectra of Silicate Minerals

Suga et al., Material Transactions , 2004: XANES of Si Nitride Thin Film by Pulsed Laser Deposition

XANES features of various silicates with a steep main absorption peak shifting by about2.5 eV from Fosterite at 1.844 keV to Quartz at 1.846 keV

Atomic Si has a low energy edge at 1.839 keVwith a shallow near absorption feature and slowrecovery; XANES features of various nitrides have main absorption peaks between atomic Siand silicates.

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Chandra HETG LMXB Si Survey: Observations 40 Observations from the Chandra Archive with9 Bright LMXB located in or near the Galactic Bulge

Source fluxes range from 5 mCrab (4U 1624-49) toabout 1 Crab (GX 5-1)

Two data modes available:

Timed Event (TE) mode: Exposure is taken in data frames with a frame time in this sample varying from about 0.9 to 1.7 sec. At these high fluxes pileup effects can still be up to 35%, but can be mitigated to < 20% in the worst case by omitting data from back illuminating devices.

Continuous Clocking (Mode): Exposure is quasi-continuous by reading out each row reducing the frame time to 3 msec. At these read out times there is no pileup but the edge can be filled in by contributions from a dispersed scattering halo. Here only data from back illuminated devices are of use because there is no instrument edge.

Consequence for both modes: The measured optical depths are likely lower limits and a systematic uncertainty of 20% has to be added. Absorptions features are generally unaffected

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Chandra HETG LMXB Si Survey: K Edge Examples

Si K edge examples in four different bright sources, over-plotted a simple edge model from Verner et al. (1995)

In excess to the step function, there are two resolved absorption features: one near, one far from the edge.

The detailed edge morphology appears different all four sources

The edge morphology cannot be fully reproduced by the edge model used in Ueda et al. (2005) in all cases

GX 5-1 GX 13+1 GX 17+2 4U 1728-34

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Chandra HETG LMXB Si Survey: K Edge Identifications

Atomic Si

Si Nitrides

Silicates Main

Silicates Secondaries

Si XII

Si XIII

Average Si K edge value: 1.844+/-0.001 keVAverage near edge absorption peak at 1.850+/-0.002 keVAverage far edge absorption peak at 1.865+/-0.001 keV

Dust Ions

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Chandra HETG LMXB Si Survey: GX 5-1

-- GX 5-1 is the brightest object in the sample and has one TE, 5 CC mode observations; we only use CC mode here

-- Obsid 5888 exhibits near edge (NE) absorption with an EW of 7.4+/-0.4 mA, far edge (FA) absorption of 5.6+/-0.3 mA

-- Obsids 10691-4 exhibit NE absorption with and <EW> of 4.5 +/-0.8 mA , FA absorption < 1 mA, and a Si XIV resonance

GX 5-1: Obsid 5888 GX 5-1: Obsids 10691-4

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Chandra HETG LMXB Si Survey: GX 13+1

-- GX 13+1 is moderately brightest and has 5 TE, 1 CC mode observations at very similar X-ray fluxes

-- All observations exhibit (NE) absorption with an EW ranging from 1.8+/-0.3 to 7.2+/-0.4 mA, far edge (FA) absorption from below detection to 4.5+/-0.4 mA

-- Ueda et all. (2004) reported on wind outflows and H-like resonance absorption. All observation show Si XIV at variable strength

Obsid 2708 Obsid11814 Obsid11815

Obsid 11816 Obsid 11817 Obsid 11818

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Chandra HETG LMXB Si Survey: Structure variability

NE Absorption

The equivalent widths (EWs) of NE absorption features rangebetween the detection limit of 0.6 mA and 8 mA. Most observations exhibit NE absorption features above detection. The EWs show that within sources NE absorptionIs variable well above the expectation from the applicationof different detector modes. In many cases we observe varia-bility within the same detector mode and at very similar X-ray fluxes

FE Absorption

The equivalent widths (EWs) of FE absorption features rangebetween the detection limit of 0.6 mA and 6 mA. (4U 1624-49 only has one observation (Xiang et al. 2011).More observations have FE absorption close to the detection limit.. The EWs show that within sources FE absorption is again highly variable.

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Chandra HETG LMXB Si Survey: Optical Depths

Case A: Case B:

Hydrogen equivalent absorption: Tbnew * powerlaw (5 – 9 A, exclude 6.5 – 7.5 A)

Case A: optical depth is determined through a simple step edge function excluding the NE absorption feature. The depths indicate that for NH > 3x1022 cm-2 silicon is significantly overabundant with respect to the ISM solution (Wilms, Allen, McCray 2000)

Case B: optical depth is determined through evaluating top and bottom edge fluxes including the NE absorption feature. The depths now indicate an overabundance with respect to the ISM solution (Wilms, Allen, McCray 2000) which is consistent with predictions for the Galactic Bulge by Draine 2003. Note the variability in these depths is significantly lower than observed in the EWs!!

Case B: optical depth is determined through evaluating top and bottom edge fluxes including the NE absorption feature. The depths now indicate an overabundance with respect to the ISM solution (Wilms, Allen, McCray 2000), which is consistent with predictions for LMXB dust scattering in the Galactic Bulge by Draine 2003.

Note the variability in these depths is significantly lower than oberved in the EWs

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Chandra HETG LMXB Si Survey: Summary and Conclusions The Si K edges in absorbed Galactic Bulge LMXBs show distinct structure consisting of two broad line components, a near edge broad line component, and a far edge broad line component. While this is consistent with the findings by Ueda et al. (2005), the interpretation and model thereof is not.

We measure an average edge value of 1.844+/-0.001 keV, the near edge broad line appears at 1.849+/-0.002 keV, and and the far edge line component at 1.865+/-0.002 keV. The edge value appears consistent with absorption by most silicates (see also Ueda et al. 2005), the corresponding near edge absorption peak, however, in some cases appears beyond the expected location of a-quartz. The far edge absorption in almost all cases is not shallow as assumed by the Ueda et al. model, appear as an absorption line, and at times very strong. This is inconsistent with the adopted silicate model.

The edge itself appears fairly constant, including the average depth of the near edge absorption feature. Near and far edge absorption in general is highly variable. Variability on the observed time scales is inconsistent with dust origins.

Based on these observations we conclude that the Si K edge is produced largely by dust, but shows interference by absorption from ionized atomic Si close to the X-ray source. The near absorption peak is then a mix of dust and a variable ionized Si XII component, the far edge absorption is identified as Si XIII resonance absorption.

The optical depths of the Si K edges produce lower limits of optical depths that indicate a significant overabundance of silicon in all these systems. In that respect dust columns are mostly > 1023 cm-2 and appear at least consistent with predictions by Draine (2003) for the Galactic Bulge in general.

The variable ionized component is then a byproduct of ionization processes close to the X-ray sources

Extensive and repeated observations are needed with ASTRO-H in order to further separate and characterize the dust component of the Si K edge

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Chandra HETG LMXB Si Survey: References

Sant’Anna et al., Phys Rev Letters, 107, 3, 2011 Costantini et al., A &A, 539, 32, 2012 DeVries & Costantini, A &A, 497, 393, 2009 Draine, ApJ Main, 598, 1026, 2003 Gatuzz et al., ApJ Main, 768, 60, 2013 Garcia et al. , ApJ Main, submitted, 2014 Juett et al. , ApJ Main, 612, 308, 2004 Juett et al. , ApJ Main, 648, 1066, 2006 Kaastra et al., A & A, 497, 291, 2009 Kortright & Kim, Phys Rev B, 62, 12216, 2000 Lee et al. , ApJ Letters, 554, L13, 2001 Lee et al., ApJ Main, 567, 1102, 2002 Lee & Ravel, ApJ Main, 622, 970, 2005 Lee et al. , ApJ Main, 702, 970, 2009) Li et al., Phys. Chem. Minerals, 22, 115, 1995 Miller et al. ApJ Main, 606, 131, 2004 Paerels et al., ApJ Main, 546, 338, 2001 Pinto et al. , A & A, 521, 79, 2010 Pinto et al. , A & A, 551, 25, 2013

Schulz et al. , ApJ Main, 565, 1141, 2002 Schulz & Brandt, ApJ Main, 572, 971, 2002Schulz et al., ApJ Main, 725, 2417, 2002Suga et al., Material Transactions , 45/7, 2039, 2004 Takei et al. , ApJ Main, 581, 307, 2002 Yao&Wang, ApJ Main, 624, 751, 2005 Yao&Wang, ,ApJ Main, 641, 930, 2006 Yao et al. , ApJ Letters, 653, L121, 2006 Yao et al. , Ap J Main, 690, 143, 2009 Ueda et al., ApJ Main, 620, 274, 2005 Wang et al. , ApJ Main, 635, 386,2005 Watanabe et al., ApJ Letters, 597, L37, 2003 Wilms, Schulz, Nowak et al. , in prep. 2014 Wilms, Lee, et al., in study, 2015

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